1. Field of the Invention
The present invention relates to an image reader (such as an image scanner) and an imaging apparatus (such as a digital photocopier or a facsimile machine) for producing an image based on the outputs from the image reader, which are capable of reducing vibration and fluctuation due to a shaking of or an impact externally applied to the light receiving elements of the image reader.
2. Description of the Related Art
In general, an image reader furnished in currently used image scanners or digital photocopiers scans-original images using a light beam. The light image reflected from the original is received, via an optical transfer system, at the light-receiving unit, and then it is converted to electric signals. The light-receiving unit generally includes an imaging lens, an image sensor, and a circuit board for processing the output from the image sensor.
To assemble the light-receiving unit into an image reader, the components constituting the light-receiving unit are optically arranged and held on a stay member (that is, a base), and the stay member holding the components on it is attached to the frame of the image reader. The structural unit including the stay member is referred to as a light-receiving block.
In general, the stay member is a long and narrow plate. Accordingly, with external vibration or impact propagating through the frame of the image reader to the light-receiving block, oscillation is likely to occur in the vertical direction with both ends of the stay member as nodes attached to a frame. The light-receiving block swings as the stay member bends, and the position of the image sensor relative to the scan beam reflected from the original changes. This results in the output images from the sensor fluctuating.
To overcome this problem, it has been proposed to provide a balance weight to the stay member (or the optical block) to restrain vibration and swinging.
FIG. 1A illustrates a conventional light-receiving block furnished with a balance weight 114. Both ends of the long stay plate 109 are attached to the frame 110 of an image reader, and a light-receiving unit comprising a lens 107 and an image sensor 108 is placed in the middle of the stay plate 109. A balance weight 114 is fixed to the rear face of the stay plate 109 corresponding to the loading position of the light-receiving unit. With external shake or impact, the light-receiving unit slightly oscillates in the directions indicated by the arrow.
Since the balance weight 114 changes the characteristics of the oscillating system, the oscillation frequency is reduced as compared with a light-receiving block elastically held by the frame 110 without the balance weight 114. The oscillation amplitude is also reduced to a certain extent.
A similar known technique for reducing the oscillation of the light-receiving unit of an image reader is to reduce the amplitude of the resonant vibration of the base plate of the image reader, on which a light-receiving unit is attached. With this technique, a balance weight is fixed to the base plate via an elastic material to absorb the vibration due to the resonance of the base plate.
However, providing a balance weight to the long and narrow stay plate 109 as shown in FIG. 1A is insufficient for the trend toward high image quality because the long plate easily oscillates with large amplitude.
Another problem in the conventional structure is shown in FIG. 1B, which is a cross-sectional view taken along the Cxe2x80x94C line in FIG. 1A. The center of gravity of the light-receiving block resides at point G, which is offset from the stay plate 109. Due to this imbalance, the light-receiving block swings in the mode to which the rotation about the axis passing through fulcrums (i.e., both ends of the stay plate 109) is added, as indicated by the curved arrow in FIG. 1B. Consequently, the balance weight 114 can not sufficiently reduce the vibration and swinging. This adversely affects the quality of the sensor output.
The present invention was conceived to overcome the above-described problems in the prior art techniques concerning oscillation and swinging due to shaking or impact externally applied to the light-receiving unit of an image reader. It is one of the objects of the invention to realize reliable damping of undesirable oscillation and swingig with a simple structure even if a long and narrow stay plate is used to hold the light-receiving unit. Such an oscillation reducing (or a damping) structure is applicable to an image reader (e.g., an image scanner) and an imaging apparatus (e.g., a digital photocopier or a facsimile machine).
To achieve the objective, in one aspect of the invention, an image reader comprises a light source illuminating an original, an optical guide system configured to guide a light beam reflected from the original and conveying information about an image on the original, a light-receiving unit configured to receive the light beam guided by the optical guide system and to convert the information about the image into electric signals, a stay member holding the light-receiving unit, a balance weight configured to act on the stay member and to reduce oscillation of the light-receiving unit, and a balance-weight holding structure configured to hold the balance weight in an unrestrained state with respect to the stay member.
In another aspect of the invention, an image reader comprises a light source illuminating an original, an optical guide system configured to guide a light beam reflected from the original and conveying information about an image on the original, a light-receiving unit configured to receive the light beam guided by the optical guide system and to convert the information about the image into electric signals, a stay member holding the light-receiving unit, the stay member and the light-receiving unit constituting a light-receiving block, and a balance weight provided to the stay member in an unrestrained manner and configured to bring the center of mass of the light-receiving block to the center along the longitudinal axis of the stay member.
Either of the above-described image readers is applicable to an imaging apparatus. In this case, stable and reliable output signals can be obtained from the image reader, and therefore, reproduced images have a high image quality without periodic streaks or fluctuations in the images.